yogurt cultures question

[Planomom2]

Hello,

My third grade daughter and her friend are working on the yogurt starter cultures project. We've finished the experiment and we're trying to "massage" the question/problem. We're literally having trouble wording it to flow well and reflect the the "crux" of the experiment.

To give you a little more background, we made homemade yogurt by heating whole milk to 185 F and then cooling it to 110 F and adding various purchased (yogurt) starter cultures to the different batches of cooled milk. We then incubated the containers of the milk mixtures for 7 hours. We definitely observed differences in taste and firmness among the different final yogurt products, which we have made extensive notes on.

Here's the problem/question they have so far:

"Making homemade yogurt, how do different starter cultures affect the taste and firmness of the yogurt?"

I know it's a little rough right now, but I'm also trying not to just write the question for them. Also, does this get "to the root" of our experiment?

Any advice you can provide would be great.

Thanks,

[deleted-71670]

Seems like a good question to me!

[donnahardy2]

Hi,

This is an excellent experiment. I think you are doing this project:

https://www.sciencebuddies.org/science- ... p010.shtml

Bacteria utilize the sugar in the milk to produce lactic acid, which lowers the pH of the milk, and this solidifies the milk. Did you notice the link in the project outline for the cabbage pH indicator:

http://www.vickicobb.com/scienceyoueat.html

You could use this to test the pH of the milk and yogurt. This would give your 3rd grader a visual confirmation of the chemical change that has occurred.

Your experiment is controlled because you have used the same milk for all of your cultures and you incubated them for the same amount of time. The difference in the quality of the yogurt you produced could be due to:

1. A different number of bacteria in the starter
2. A different the species of bacteria used
3. A difference in the growth rate of the bacteria

Here are two websites that include background information for this project.

http://en.wikipedia.org/wiki/Yoghurt

http://en.wikipedia.org/wiki/Lactobacillus

Here is a photomicrograph of bacteria from yogurt:

http://www.bitelog.com/images-items/pfo ... 5_0009.jpg

Donna Hardy